1. Field of the Invention
The present invention relates to a data processing apparatus and a processing method for preventing the use of digital data for illegal (in justice) activities, and to a storage medium on which the data processing method is stored.
2. Related Background Art
Compared to conventional analog data, digital data is not easily degraded when it is copied or altered by a computer, and it is easily transmitted over a communication line. Because of these characteristics, the illegal copying and redistribution of digital data tend to occur fairly often.
One method that is used to prevent these illegal activities is called digital watermarking. According to this method, digital watermark information is embedded in such a manner that when digital contents having an embedded digital watermark are normally reproduced, the watermark can not be discerned by an observer.
For the purposes of this document, digital contents are static pictures, moving pictures, sounds, computer programs and computer data.
A typical method used for embedding digital watermark information is one whereby, to embed a digital watermark, the level of brightness or the chromaticity of each pixel constituting a color image is computed.
The method disclosed in U.S. Pat. No. 5,636,292, for example, is a well known example of the above method. According to this method, digital contents are divided into blocks, and a watermark pattern that includes the coefficients +1 and −1 is added to each block to implement the embedding of a digital watermark.
Another well known embedding method is one for which frequency transformations, such as fast Fourier transforms, discrete cosine transforms or wavelet transforms, are performed, and for which digital watermark information is added to frequency areas and inverse frequency transformations are used.
According to the method for which fast the Fourier transforms are used, a PN series is added to input digital contents and the resultant digital contents are distributed and divided into blocks. A Fourier transform is performed for each block, and one bit of watermark information is embedded in one block. An inverse Fourier transform is performed for a block in which the watermark information is embedded, and the same PN series as was added at first is added to the block, so that digital contents in which a digital watermark has been embedded can be obtained. This method is described in detail in “Watermarking Method For Images Using PN Series”, Onishi, Oka and Matsui, 1997 Cryptography and information security symposium lecture theses, SCIS97-26B.
According to the method for which the discrete cosine transforms are used, digital contents are divided into blocks, and a discrete cosine transform is performed for each block. Then, one bit of watermark information is embedded in one block, and an inverse transform is performed for the block to generate digital contents in which a digital watermark has been embedded. This method is described in detail in “Digital Watermarking Method In Frequency Area To Protect Copyright of Digital Image,” Nakamura, Ogawa and Takashima, 1997 Cryptography and information security symposium lecture theses, SCIS97-26A.
According to the method for which the wavelet transforms are used, it is not necessary to divide into blocks the digital contents that are input. This method is described in detail in “Experimental Study Concerning Safety And Reliability For Digital Watermarking Technique Using Wavelet Transform”, Ishizuka, Sakai and Sakurai, 1997 Cryptography and information security symposium lecture theses, SCIS97-26D.
The typical digital watermark information that is embedded using the above methods is copyright information and user information. When copyright information has been embedded in original digital contents, users thereafter not only know that the pertinent digital contents are copyrighted but also know the name of the copyright owner.
Further, when user information has also been embedded, a user who illegally redistributes the pertinent digital contents can be identified.
The conventional digital watermarking technique, however, is a passive protection measure, merely imparting information concerning the prohibition of an illegal act, and a method that can actively and effectively prevent or detect an illegal act has not yet been established.